1. Technical Field
This application is related to alloys with controllable compositions and physical properties, including optoelectrical and mechanical properties, their use in optoelectronic devices and methods of making such alloys.
2. Description of the Related Art
Optoelectronic devices include a wide range of electrical-to-optical, or optical-to-electrical transducers, such as photodiodes (including solar cells), phototransistors, light-dependent resistors, lasers, light-emitting diodes (LED), fiber optics and the like. Regardless of the type, an optoelectronic device operates based on at least one of two fundamental processes, namely, creating electron-hole pairs by photon absorption, or emitting photons by recombining electrons and holes.
Semiconductor materials have unique electronic band structures, which can be impacted by the quantum mechanical effect of light. They are thus materials of choice in fabricating optoelectronic devices. In a semiconductor material, the uppermost-occupied band is typically completely filled and is referred to as a valence band; whereas the lowest unoccupied band is referred to as a conduction band. Electrons in the valence band can absorb photon energy and be excited to the conduction band, leaving holes in the valence band. The semiconductor material becomes conductive when an appreciable number of electrons are present in the conduction band. Conversely, electrons in the conduction band can be recombined with a hole in the valence band and cause spontaneous or stimulated emission of photons.
The optical and electrical properties of a semiconductor material are largely determined by the energy difference (“band gap”) between its valence band and conduction band. For example, during the process of creating electron-hole pairs, the bandgap is a direct measure of the minimum photon energy required to excite an electron from the valence band to the conduction band. When an electron and hole recombine, the bandgap determines the photon energy emitted. Accordingly, controlling the bandgap is an effective way of controlling the optical and electrical properties and outputs of the optoelectronic devices.
The bandgap is an intrinsic property of a given semiconductor material. Bandgaps can be adjusted by doping a semiconductor material with an impurity according to known methods. Alternatively, semiconductor alloys formed by two or more semiconductor components have been created. The bandgap of such an alloy is different from that of the semiconductor components, and is typically a function of the bandgaps and the relative amounts of the components.
Generally speaking, in creating a new alloy, two or more elements are allowed to grow into one crystal lattice. More commonly, two types of binary alloys (e.g., AlAs, InP, GaAs and the like) are grown into a tertiary or quaternary alloy. Lattice match of the components is therefore important in reducing the strain and defects of the resulting alloy.
FIG. 1 shows the bandgap energies (eV) and lattice constants of various Group III-V semiconductors. As illustrated, two binary semiconductor alloys, AlAs and GaAs, have nearly identical lattice constants (about 5.65). Their bandgaps are respectively 2.20 eV and 1.42 eV. Because of the matching lattice constants, AlAs and GaAs are suitable to form a relatively stable tertiary alloy, which can be represented by AlxGa1−xAs (x being the atomic percentage of AlAs in the alloy). The bandgap of the tertiary alloy is a function of x as well as the bandgaps of the pure AlAs and GaAs. This example illustrates an approach to engineering bandgaps by controlling the compositions of semiconductor alloys.
Controlling the composition of an alloy shows promise for creating new materials with tunable optoelectrical or mechanical properties. Currently, semiconductor alloys such as AlxGa1−xAs, InxGa1−xN and AlxGa1−xN are fabricated by epitaxial growth techniques such as Metal Organic Chemical Vapor Deposition (MOCVD) or Molecular Beam Epitaxy (MBE). However, technical challenges remain in growing these epitaxial layers, in spite of the relative strain-tolerance and defect-tolerance of the materials. In particular, their mechanical stability and integrity are difficult to maintain due to strain, which, in turn, limits the thickness of the layers grown. The compositional control is also influenced by the strain in the material.
Some semiconductor materials do not have an acceptable lattice match that will permit them to be formed in a stable compound or heterostructure using standard bulk crystal or epitaxial growth techniques. Thus, engineering a specific bandgap or having a particular alloy composition is very difficult and sometimes not possible with current semiconductor technology